1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a laser beam printer or a facsimile machine.
2. Related Background Art
Hitherto, since an image forming apparatus, such as a copying machine, that is able to form images on two sides of sheets, causes the sheet to straight pass through a transferring portion thereof and prevents excess change in the quantity of formed curl (hereinafter called "curl") depending upon the quantity of toner, the image forming apparatus of the foregoing type has been used relatively early. The schematic structure of the foregoing two-side copying machine will now be described. A sheet supplied from a sheet cassette or the like is carried by a transferring drum so that an image is formed and fixed on the first side of the sheet. Then, each sheet is reversed to cause the side of the sheet to face upwards and the sheets are stacked on an intermediate tray, and then the sheets are again sequentially supplied so that images are formed on the second sides of the sheets.
Among the above-mentioned conventional two-side copying machines, a color copying machine, in particular, an apparatus of a type, in which a sheet is carried by a transferring drum and thus image transference is performed plural times, however involves reduction in water in the sheet, on which an image has been fixed after the first image transference, causing the sheet to be hardened. If the quantity of toner is large, the sheet will easily be curled, causing the sheet conveyance operation to be made unstable at the time of the second image forming operation. Thus, the reliability in stacking the sheets on the intermediate tray is unsatisfactory and the sheet cannot sometimes stably be wound around the transferring drum.
That is, as shown in FIG. 21, if sheets 302, to each of which an image for the first side has been fixed, and which have been reversed and conveyed, are curled downwards, following sheets 302 conveyed in a direction indicated by an arrow shown in FIG. 21 are undesirably introduced into the stacked sheets 302. As a result, the order of pages will be disordered and the sheets cannot be stacked as desired, thus causing paper jamming to take place. To prevent this, it might be considered feasible to employ a method in which the sheets 302 are curled upwards. However, undesirable downward curl is formed depending upon the quantity of toner and thus a satisfactory preventive effect has not been obtained. In a case where the sheets 302 are curled upwards, the sheets 302 cannot be easily wound around the transferring drum because the direction of the curl prevents the sheets 302 from being easily wound around the transferring drum when the sheets 302 are again supplied from the intermediate tray 301.
As a result, the sheets cannot smoothly be conveyed when a predetermined number of sheets are stacked in a sheet staking portion. An apparatus of a type, in which sheets are again supplied to the image forming portion after the sheets have been stacked, confronts a problem in that warp of the sheets causes a problem in conveying the sheets and involves a defective image quality, such as incomplete image transference.
Since an ordinary sheet carrier (the transferring drum) is formed into a cylindrical shape, sheets can stably be wound if the sheets are previously curled in the direction of the circumference of the cylindrical shape. In a case where the sheets are electrostatically adsorbed around the transferring drum, the previous curling of the sheets is significantly effective. If only the leading portion of the sheet is previously curled, the sheets can stably be carried.
Since no water reduction takes place and no toner adheres to the sheet when images are formed on the first sides of the sheets, thin sheets can be carried stably even if the sheets are curled slightly or the sheets are not curled because the thin sheets (100 g/m.sup.2 or less) have weak stiffness. In a case of thick sheets, the thick sheets can be carried stably or electrostatically adsorbed by previously curling the sheets. Thick sheets can be usually fed from a manual paper feeding tray and are allowed to pass through curl-forming rollers so as to be curled. Thick sheets can be curled by the rollers more strongly than thin sheets. The reason for this is that deflection of fibers of the paper occurring when the thick sheet passes between the curl-forming rollers cannot easily be restored as compared with thin sheets. Thin sheets are smoothly guided in a direction of curling and therefore the quantity of the curl is reduced. Since thin sheets are stably carried or electrostatically adsorbed even if the quantity of curl is small or no curl is formed, the foregoing curl forming characteristic realized by using the curl-forming rollers is satisfactory. Since even thin sheets have stiffness at the second transferring operation and the sheets are upwardly curled when the sheets have been stacked on the intermediate stacking means as shown in FIG. 22 in a case where an image is formed on each of the first side of the sheets by using toner in a small quantity of adhesion. Therefore, it is advantageous to prevent jamming at the time of stacking the sheets. However, it is disadvantageous to carry the sheet or to electrostatically adsorb the sheet to the transferring drum at the second transference operation. Therefore, even if the sheets are allowed to pass through an adequate curl-forming means at the time of the first transference operation, the sheets cannot be carried by the transferring drum or electrically adsorbed.